Castrol Magnatec any day.
The Mobil Super S, whilst claiming to be a 'semi-synthetic' is actually just a refined mineral oil, using Group3 basestocks, also known as 'synthesised hydrocarbon fluids' (SHFs). It is illegal in the EU to even refer to any Gp3 SHFs as 'synthetic'. Oh, and it has no VW petrol engine approval either.
The Castrol Magnatec is a superb and much under-rated oil. Like the Mobil Super S, the Castrol Magnatec is a semi-synthetic - but a true semi-synthetic. It uses extremely high quality Group5 basestocks, aka 'esters'. These esters are actually 'polarised' (similar to being electrically charged) - and just like their advertising, work very similar to a magnet, and actually 'cling' to metal surfaces, rather than draining back into the sump overnight. Just make sure you dont use the 5w30 Magnatec, because that will be too thin. You will need either the 10w40, or if you have done some serious mileage (over 100k miles) then the 15w40 Magnatec would be better.
BTW, what is your engine?
HTH